The present invention relates to a device for measuring gas flow-rate, particularly for burners.
In known devices, the flow-rate of a gas is generally calculated with the use of a xe2x80x9chot-wirexe2x80x9d sensor, that is, a wire resistor which is heated to a predetermined temperature by a current and is positioned in the gas-flow. By xe2x80x9cpredetermined temperaturexe2x80x9d is meant a reasonably predictable, as opposed to random, temperature determined before the wire resistor is heated. The flow-rate of the gas is obtained by known physical laws, by measuring the power dissipated by the sensor.
Since the value of the power dissipated is dependent not only on the flow-rate but also on the temperature of the gas, the signal output by the sensor is generally compensated to account for the possible variations of this temperature. For this purpose, a xe2x80x9ccoldxe2x80x9d sensor, also positioned in the gas-flow, is typically used to measure the temperature thereof.
However, a temperature-compensating circuit comprising the sensors mentioned is not easy to design and often requires complex circuit arrangements. Moreover, the cost of the sensors used is usually quite high since the signal output by the sensor also depends on the constructional characteristics of the sensor itself. In order to obtain the same flow-rate measurement from two devices including two different flow-rate sensors, it is therefore necessary for the two sensors to have very similar characteristics.
In addition, in devices according to the prior art, the two sensors for measuring flow-rate and temperature are generally positioned in a single probe to be inserted in the duct through which the gas-flow to be measured is flowing. The introduction of this probe causes a pressure drop in the duct which may lead to malfunction of the apparatus to which the gas is supplied.
German patent application No. DE-A-19855870 and European patent application No. EP-A-0838723 disclose a measuring device. The technical problem underlying the present invention is that of providing a flow-rate measuring device, particularly for burners, which is designed structurally and functionally to prevent the problems discussed with reference to the prior art mentioned.
The present invention solves the problem posed with a flow-rate measuring device. The device according to the present invention measures the flow-rate of a gas-flow in a duct and includes a gas flow-rate sensor which can generate a first output signal proportional to the flow-rate detected. The device also includes a temperature-compensation circuit to which the first output signal is applied and which can generate a second output signal proportional to the flow-rate of gas detected by the flow-rate sensor and independent of one or both of the temperature of the gas and the temperature of the flow-rate sensor. The compensation circuit has a temperature sensor. The second output signal of the temperature-compensation circuit is applied to a calibration circuit.
The behaviour of the second output signal is represented, for a fixed flow-rate, by a family of curves of known equation depending on the parameters of the flow-rate sensor. The calibration circuit includes a first and a second resistive network of variable overall resistance by which each curve of the family representing the second output signal is made to pass through a first and a second predetermined fixed point, so that the calibration circuit is able to generate a third output signal proportional to the gas flow-rate detected and independent of structural parameters of the flow-rate sensor and/or of the temperature sensor. By xe2x80x9cpredetermined fixed pointsxe2x80x9d is meant a reasonably predictable, as opposed to random, fixed point determined before the family of curves is drawn. Thus, the third output signal is correlated with the flow-rate detected and independent of the temperature of the gas and/or of the temperature of the flow-rate sensor as well as of the structural parameters of the flow-rate sensor.